1. Field of the Invention
The present invention relates to a method for slag coating of a converter wall, being intended to extend the life of the converter. The present invention relates also to a method for controlling the thickness of the converter bottom, which tends to increase as a result of repeated slag coating onto the converter wall.
According to the present invention, slag coating is accomplished by blowing out a gas from a top-blown lance so that slag is splashed uniformly onto the barrel and throat near the trunnion of the converter. Slag coating in this way makes it possible to repair the bottom and wall, unlike conventional slag coating which is carried out by tilting the converter.
2. Description of the Related Art
Among conventional methods for repairing the bottom and wall of converters is slag coating. It is designed to utilize slag (resulting from refining) for protection of the bottom and wall refractories of the converter, ready for the next run. It can be applied to both top-blown converters and top-bottom-blown converters, and it is generally used as a convenient rapid repair method. (See Japanese Patent Laid-open No. 37120/1978.)
To apply this repair method, the converter is tilted to discharge refined steel and slag in such a way that at least part of molten slag remains, in the converter. Then, the remaining slag is combined with dolomite (as a solidifier) and the converter is swung around the trunnion axis such that slag sticks to the bottom and wall refractories of the converter. The slag solidifier increases the melting point of slag and decreases the flowability of slag, thereby making the slag stick easily. The disadvantage of this method is that the slag does not stick sufficiently to the area below the position near the trunnion (referred to as the trunnion a side hereinafter) which remains a dead zone when the converter is swung. Hence protection of refractory by the slag is not accomplished.
To address this problem, a new method of slag coating was proposed in Japanese Patent Laid-open No. 16111/1982. This method consists of blowing an inert gas through the bottom-blown nozzle such that the remaining slag in the converter is blown up by the inert gas and caused to stick to the wall refractories. (This method is applicable to both bottom-blown converters and top-bottom-blown converters.) In this way it is possible to apply slag to the bottom and wall below the trunnion side. The disadvantage of this method is the difficulty in splashing slag in desired directions and in distributing slag uniformly on the wall refractories despite the blowing of inert gas at a controlled flow rate.
The present inventors proposed in Japanese Patent Laid-open No. 41815/1995 a method of slag coating which involves the blowing of inert gas through a top-blown lance (in place of bottom-blown nozzles) in top-blown converters and top-bottom-blown converters. This method permits slag coating on the trunnion side, particularly the knuckle part (the boundary between the bottom and the wall) and the bottom, which are difficult to repair by a conventional method. According to this method, an inert gas is blown such that slag is moved to the wall and caused to crawl up along the wall. Slag coating in this way is limited in coating area and is poor in uniformity of coating on refractories. Another disadvantage is incomplete slag coating on the barrel near the trunnion side, and difficulty in coating up to the throat. Therefore, slag coating in this way is not an adequate method of repairing converters.
As mentioned above, Japanese Patent Laid-open No. 37120/1978 discloses a method of slag coating by causing part of molten slag to remain in the converter, adding a solidifier to it, swinging the converter around the trunnion axis, and causing slag to stick to the bottom and wall refractories. The disadvantage of this method is incapability to repair the trunnion side.
Japanese Patent Laid-open No. 16111/1982 discloses a method of slag coating by splashing upward residual slag in the converter with an inert gas blown through the bottom nozzles, thereby causing slag to stick to the wall refractories. The disadvantage of this method is difficulty in splashing slag in desired directions.
Japanese Patent Laid-open No. 41815/1992 discloses a method of slag coating by adding a solidifier to remaining slag, blowing an inert gas through a top-blown lance so as to move slag toward the wall, thereby causing slag to stick to the wall refractories. The disadvantage of this method is the limited coating area, the lack of uniformity in coating, and the difficulty in controlling the slag properties by controlling the lance height and gas flow rate, and also by the addition of a solidifier.
The present invention was completed to address the above-mentioned problems involved in the prior art technologies.
It is an object of the present invention to provide a new method for slag coating on the converter wall to extend the life of the converter.
According to this method, slag coating is accomplished by blowing a gas from a top-blown lance in a special way toward slag remaining in the converter after tapping in such a way that slag is splashed and stuck to the converter wall. During this slag coating, slag properties are well controlled by adding a slag solidifier and splashing slag is controlled by adjusting the lance height and the gas flow rate, so that the blown slag uniformly and stably sticks to the converter wall, including the barrel, trunnion side, and throat which could not otherwise be repaired by conventional slag coating by tilting the converter.
It is another object of the present invention to provide a method for limiting and controlling the thickness of the converter bottom which would otherwise increase due to accumulation of solidified slag after repeated slag coating onto the converter wall. The method of this invention permits detection of any such increase.
We have carried out extensive studies to find a solution to the above-mentioned problems, by studying the conventional method of slag coating, which consists of causing molten slag to remain on the bottom of the converter after tapping and blowing a gas from a top-blown lance, such that the molten slag is splashed and stuck to the converter wall. As the result, we found that uniform slag coating over the entire surface of the converter wall can be achieved if the lance height (from the bottom) and the gas flow rate are critically adjusted so that the slag is splashed to the desired part of the furnace that needs repair and, immediately after or a certain period after the start of inert gas blowing, the slag is combined with a slag solidifier containing MgO or CaO which forms solid slag in a critical ratio, combined with adjusting the splash height and stickiness of the slag.
In accordance with this invention, the molten slag is caused to remain on the bottom of the converter after tapping and blowing a gas from a top-blown lance, thereby splashing the molten slag and sticking the molten slag to the converter wall, characterized in that the lance height measured from the bottom is adjusted to about 0.7-3.0 m and the gas flow rate is adjusted to about 250-600 Nm3/min and, after gas blowing, the remaining molten slag is combined with a slag solidifier containing MgO or CaO according to its composition, and top blowing in the presence of the slag solidifer so that the height of slag splashing is controlled in the presence of the slag solidifier and the amount of slag sticking to the converter wall is controlled and the molten slag solidifier mixture is splashed toward the desired part of the converter wall that needs repair.
An important feature of this invention resides in the lance height from the bottom being adjusted to about 1.0-3.0 m and the gas flow rate adjusted to about 250-600 Nm3/min and, after gas blowing, the remaining molten slag is combined with a slag solidifier containing MgO or CaO according to its composition in an amount enough for the ratio of solid phase in the slag to reach about 0.50-0.70.
In a preferred embodiment, the slag solidifier is added to the remaining slag about 0-2 minutes after the start of gas blowing.
In another preferred embodiment, the slag solidifier is added in combination with all reducing agent so that the ratio of solid phase in the slag is increased to about 0.50-0.70 in the case where the oxygen potential in slag is higher than about 22% in terms of T.Fe.
T.Fe=Total iron content in slag (%), which means metallic iron and iron as oxide(FeO, Fe2O3, Fe3O4, etc all the type).
The gas used for slag splashing may be an inert gas such as nitrogen, argon, or a mixture thereof, or air or a mixture containing air.
The gas flow rate may be reduced to about 250 Nm3/min if the part to be repaired is lower than about 3 meters from the bottom of the converter. The gas flow rate may be increased to about 600 Nm3/min if the part to be repaired is higher than about 7 meters from the bottom of the converter. In other words, the gas flow rate may be adjusted to save utility cost according to the position of the part to be repaired.
It is advantageous to control the bottom thickness of the converter by detecting the back pressure of the gas being forced into the converter through a bottom-blown tuyere and to sense or measure the increase of the bottom thickness of the converter based on the increase of the back pressure at the bottom-blown tuyere.
It is also beneficial to control the bottom thickness of the converter whose wall is coated with slag, by adding an alumina source to the molten slag remaining at the bottom of the converter after tapping, thereby decreasing the melting point of the slag, and stirring the slag with a gas introduced through a bottom-blown tuyere and/or a top-blown lance.
The foregoing and other important features of the invention are shown in specific drawings that serve as examples, but are not intended to define or to limit the scope of the invention.